The present disclosure relates to a sheet conveying device for feeding sheets, and to an image forming apparatus having same.
Image forming apparatuses such as photocopiers, multifunctional peripherals, printers, and facsimiles, have sheet conveying devices to feed sheets. The sheet conveying device is provided with sensors to detect whether or not a sheet is present, so as to detect that a sheet has arrived at a predetermined point or has passed over a predetermined point. There are situations where reflective optical sensors are used for the sensors.
There is known a sheet conveying device capable of detecting sheets using reflective optical sensors. Specifically, the sheet conveying device includes a first sheet conveying unit which feeds a sheet after the leading edge of the sheet is contacted to correct skewing of the sheet; a second sheet conveying unit situated upstream in the feed direction of the first sheet conveying unit to feed a sheet fed from a sheet feed device to the first sheet conveying unit; a seat conveying unit located between the first and second sheet conveying unit; and a reflective optical sensor located near the seat conveying unit, to detect whether or not there is a sheet passing through the sheet conveying unit. With this sheet conveying device, the amount of contact of the sheet against the first sheet conveying unit by the second sheet conveying unit is determined using the detection results of the reflective optical sensor. The sheet conveying device has an upper conveying guide and a lower conveying guide which hold a sheet from both the front and back directions, positioned with a tapered form as to the feed direction. The angle between the upper conveying guide and an optical axis of the reflective optical sensor and the angle between the lower conveying guide and the optical axis of the reflective optical sensor are set to a predetermined angle enabling detection regardless of the type of sheet being fed. With this sheet conveying device, conditions relating to sheet detection, such as distance between sheet and sensor, angle between sheet and detection face of sensor, and so forth, are set so as to satisfy a range where sheets can be accurately detected, so from the leading end to the trailing end of the sheet can be accurately detected using a single reflective optical sensor.
Arrival and passage of sheets are detected using optical sensors with some image forming apparatus. Transmissive optical sensors and reflective optical sensors are used as optical sensors.
The transmissive optical sensor includes a light-emitting element, a light-receiving element, an actuator (rotating plate, fan-shaped for example) which rotates (turns) upon coming into contact with a sheet, and so forth. When there is no sheet, the actuator interrupts the light from the light-emitting element to the light-receiving element. On the other hand, when a sheet arrives, the actuator is rotated by the sheet, and the state in which light from the light-emitting element to the light-receiving element is not interrupted continues until the sheet completely passes.
On the other hand, a reflective optical sensor includes a light-emitting element which emits light toward the conveyance path of the sheets, and a light-receiving element which receives reflected light emitted by the light-emitting element. If there is no sheet in a detection range, the amount of light received by the light-receiving element from reflection is small, but if a sheet exists in the detection region (from the time that the sheet arrives until completely passing), the amount of light received by the light-receiving element from reflection is great.
Thus, both transmissive optical sensors and reflective optical sensors take advantage of the difference in the output of the light-receiving element depending on whether or not there is a sheet at the position where the optical sensor is installed (detection region), to detect whether or not a sheet currently exists at the position where the optical sensor is installed.
Now, image forming apparatuses sometimes print on transparent or semi-transparent sheets, such as sheets for overhead projectors (OHPs), hereinafter interchangeably referred to as “overhead transparency sheets” or “OHP sheets”. Accordingly, there are situations where image forming apparatuses use only transmissive optical sensors and not reflective optical sensors, since sheets, through which light can pass, can be accurately detected. On the other hand, there are some situations where reflective optical sensors are used when transmissive optical sensors cannot be used due to reasons such as insufficient space to install actuators, actuators becoming a load on sheet conveyance, and so forth.
Now, light-receiving elements of reflective optical sensors have more trouble receiving reflected light the greater the angle of the sheet (feed direction of the sheet) deviates from being at a right angle as to the optical axis of the light-emitting element. Also, the greater the distance is between the reflective optical sensor and the sheet, the more difficulty the light-receiving element has in receiving reflected light.
Accordingly, the angle between sheets being fed and the optical axis of the light-emitting element of the reflective optical sensor need to be kept within a certain angle range, so as to keep the output of the light-receiving element within an output range of when passing of sheet is detected, so as to enable detection of the sheets. Particularly, the angle between the passing sheets and the optical axis of the light-emitting element is preferably close to 90 degrees for sheets, through which light can pass, such as OHP sheets and the like.
However, there are situations where reflective optical sensors cannot be installed such that the optical axis of the light-emitting element is perpendicular with respect to the conveyance face of the sheet, due to restrictions in installation space. There are also situations where there are multiple angles of sheets passing through a detection region of a reflective optical sensor with respect to the optical axis of the light-emitting element. Accordingly, there is a problem in that there are situations where the presence or absence of sheets (OHP sheets in particular) cannot be accurately detected using reflective optical sensors. Inaccurate sheet detection results can result in sheet conveyance control, based on sensor detection results, not being appropriately carried out.
The above-described sheet conveying device has an upper conveying guide and a lower conveying guide located such that the angle of sheets with respect to the optical axis of the reflective optical sensor is kept within a certain range, and the reflective optical sensor is located at a position capable of detecting sheets regardless of the type of sheet being fed. Thus, the sheet conveying device can even accurately detect OHP sheets. However, there have been situations where whether or not a sheet is present could not be accurately detected (particularly OHP sheets), in situations where there is a limitation in the position or angle of installation of the reflective optical sensor, or situations where there are multiple patterns in sheet inclination.